At about 40.42 in the "Thoughts on static regex" there is
written "even compile-time printf would be awesome". There is a
patch about __ctWrite in GitHug, it should be fixed and merged.

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Bye,
bearophile

I wish I'd taken the mic at the end, and 2 days later Adam D.
Ruppe said what I was thinking of saying: unit test and debug the
CTFE function at runtime and then use it at compile-time when
it's ready for production.

Yes, Dmitry brought up compiler bugs. But if you write a
compile-time UT and it fails, you'll know it wasn't because of
your own code because the run-time ones still pass.

Maybe there's still a place for something more than pragma msg,
but I'd definitely advocate for the above at least in the
beginning. If anything, easier ways to write compile-time UTs
would be, to me, preferable to a compile-time printf.